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Why Do Snowflakes Come in So Many Shapes and Sizes?

In this media-rich lesson, students learn the physics of snowflake formation. They build an apparatus to grow their own snow crystals and explore how the forces that act on water molecules result in the hexagonal shapes of snowflakes.

Lesson Summary

Overview

Are all snowflakes truly unique? What are the physical forces that drive snowflakes to come in so many shapes and sizes? In this lesson, students build an apparatus that creates conditions similar to a winter cloud and produce their own snow crystals indoors. By watching the snow crystals grow, they learn about the molecular forces that shape ice crystals, and gain a deeper understanding of the states of matter. By exploring media resources, including microphotographs of real snowflakes, students also learn about molecular forces, the particulate nature of matter, and condensation.

Objectives

Understand how water vapor can condense into a solid

Understand how the forces that act on water molecules at the atomic and molecular levels result in the regular shapes of hexagonal snowflakes

Understand how snow forms in clouds

Appreciate that the different shapes of snowflakes depend upon the conditions under which they form

Grade Level: 6-8

Suggested Time

Two 1.5-hour lab periods and approximately 1.5 hours of teacher prep time before class

Multimedia Resources

Materials

Balls and sticks for making hexagonal molecular models (optional)

Dry ice (approximately 1/2 kg per student or 15 kg for a class of 30 students). Dry ice (solid CO2) is available through ice distributors in most medium- and large- sized cities. Look in the phone book or search on the Web for "Ice" or "Dry Ice." Dry ice is also used in shipping frozen materials, such as biological samples, and might be available from an ice cream shop, retail ice warehouses, a local hospital, or fish and bait shops.

Three extra-large-diameter polystyrene foam cups (or similar container that provides enough clearance around the soda bottle—which has a diameter of 2.5 inches)

Small kitchen sponge (1/2 inch thick)

Nylon fishing line (thinner is better; one-pound test is good)

Strong sewing needle

Pushpin

Four straight pins

Paper clip

Paper towels

Small magnifying glasses

Before the Lesson

Arrange computers with Internet access so students can work in pairs or small groups.

Collect all materials.

Prepare the soda bottles for students to use for their diffusion chambers. Rinse out each bottle then use a sharp knife to cut the bottles almost all the way around (leave a small hinge), about 1/2 inch above the bottom, as shown in the figure in the Instructions for Growing Your Own Snow Crystals PDF Document.

Take precautions so the dry ice does not contact bare skin.

Cut or chop dry ice into gravel-sized pieces. One way to do this is to put the dry ice inside two plastic grocery bags, and pound on it with a hammer (or other blunt object) to crush it. This works best on a hard surface, like concrete or asphalt. Dry ice is much softer than water ice, and it crushes very easily. Put the crushed dry ice back into its cooler. You can then use a spoon to transfer some into the polystyrene foam cups around the students' chambers.

Review the Instructions for Growing Your Own Snow Crystals PDF Document, which explains the process of growing snowflakes in the diffusion chambers. Then, because the results depend in large part on allowing the diffusion chamber to become cold enough to form the crystals, try the activity yourself. In particular, you should focus on allowing enough room to completely surround the chambers with crushed dry ice, and then cutting a hole for the soda bottle in a piece of cardboard and making a cover for the dry ice. This will keep the dry ice from sublimating prematurely and stretch out the time available for the experiment.

Students should already be familiar with the water cycle and the particulate nature of matter. If you feel that students need to review the water cycle, assign some readings in their textbooks about cloud formation, condensation, and precipitation, and ask them to be prepared to describe the water cycle in detail. If you feel that some students need to review the particle model, ask them to view the Particulate Nature of Matter Flash Interactive. After completing the interactive activity, students should be familiar with the molecular formula of water, H2O, and know that water molecules are formed from different kinds of atoms, one oxygen and two hydrogen. Ask students to draw the molecules, making sure that they place the oxygen atom in the middle, and the two smaller hydrogen atoms at (approximately) a 120° angle.

The Lesson

Part I: Exploring Snowflakes

1. Tell students that they will begin by viewing microphotographs of real snowflakes collected from different parts of North America. Divide the class into pairs or small groups, and have each group view the Snowflake Photo Gallery PDF Document. How many different kinds of snowflakes can they see? Do they observe any patterns?

2. Show students "Types of Snowflakes" in the Snowflake Physics Flash Interactive and ask them how they might classify the snowflakes they described earlier. Then ask students to hypothesize in their notebooks about how the different kinds of snowflakes form, including the most familiar kind, the six-sided, feathery shaped ones. You may want to ask students to draw the different stages in the development of a snowflake. When they are finished, ask them if they have ever had the chance to look at real snowflakes. Did any of them look like these?

3. Bring the class together to discuss what they've found. Most students will have noticed that many snowflakes are six-sided. Ask them if that seems to be universally true. Are there any cubical or tetrahedral snowflakes? Some students may notice that a few of the snowflakes look like feathers, cylinders, or spindles. Make note of this on the board as it will come up again later in the lesson.

Part II: Classification of Snowflakes

4. Tell students that they will now have the opportunity to watch snowflakes forming in the laboratory. Show "Watch a Snowflake Grow" in the Snowflake Physics Flash Interactive to present actual evidence of how snowflakes form. Discuss the following questions to encourage students to think about how the conditions under which snowflakes grow can affect the outcome of their structure.

Do snowflakes form by adding material or by taking it away?

What do you notice about the speed of growth of the points compared to the areas in between?

If the points grew quickly, and the areas in between grew slowly, how would that affect the final shape?

5. [Optional] Using a number of physical stick models, demonstrate the 109° angle of water molecules. Show how these can be built into a hexagon. Then have students build their own hexagons. Ask them to arrange their hexagons in different patterns that might suggest how snowflakes are formed. They can stack them and make towers, or combine them edge to edge. Once they've explored this for a while, have them view the snowflakes again and compare their ideas about snowflake formation with what they have observed. Finally, they can read the "Snowflake Basics" text in the Snowflake Physics Flash Interactive.

Part III: Growing Snowflakes in the Classroom

6. Tell students that they will now build a diffusion chamber—an apparatus that grows snowflakes. Distribute the Instructions for Growing Your Own Snow Crystals PDF Document, then have students work in pairs or small groups to create their chambers. To save time, make sure the materials are available in a central location.

7. As the snowflakes grow in the diffusion chamber, each student should make a sketch of their growing snowflakes every three minutes. One factor to note is how far down the nylon line the snowflakes are located. Encourage students to try to draw the nylon line to scale and to show all the colonies of snowflakes that are growing at different locations along the line. This time-lapse series of drawings will be used in the follow-up discussion. Remind students that the snowflakes are fragile, and that a slight tap on the bottle might dislodge them.

8. Bring the class back together so that students can share their results. Start the discussion by asking if anyone noticed hexagonal shapes or the traditional 109–120° angles. They may have to look carefully to see these, as the results are often a feathery frost, or "fuzz." Encourage them to look closely. Even students whose snowflakes did not grow to be perfectly formed will have their results validated when they see that the principles followed by the water vapor molecules as they condense hold true whether their result is needles (dendrites), traditional snowflakes, or even the fishbone pattern. Try to elicit student theories about why some of their results differed while others were the same. For example, students may have noticed that the largest flakes tended to grow at the same location on the string for each group—right above the top of the cardboard cover, where the cold air in the bottom meets the warm humid air at the top.

You might want to point out how sensitive the crystals are to the conditions of humidity and temperature under which they are formed. Even a slight difference of a few degrees will greatly alter the size and structure of the crystals. Inside the diffusion chambers, there is a gradient of temperature (warm at the top, cold at the bottom).

Inside clouds, there is also a temperature gradient. As the supersaturated air moves about inside the clouds, the growing ice crystals are subjected to different conditions. As they tumble and fall through the clouds, no snowflake follows exactly the same trajectory, nor does it experience the same conditions as any other. Thus, on the microscopic level, every snowflake is indeed different.

9. At this point, many students will be ready to explore in detail some of the physics behind snowflakes. Show students the Snow Morphology Diagram PDF Document. You may need to explain this diagram, starting with the meaning of the two axes. Temperature, along the bottom axis, varies due to weather conditions and height above Earth’s surface (generally the higher the altitude, the colder it is). Saturation can vary within a cloud based on the mixing of different air masses along a frontal system. Based on this diagram, ask students to discuss under what conditions of temperature and saturation their snow crystals would have formed if they had been formed naturally in a cloud. What could they do to their apparatus if they wanted to try to grow a different type of snowflake?

Check for Understanding

Select one of the following activities for all students to complete, or assign each activity to half of the class:

Ask students to examine the snowflakes in the A Guide to Snowflakes PDF Document and try to describe the conditions under which the different snowflakes were formed.

Have students review the "Snowflake Basics" text in the Snowflake Physics Flash Interactive. Students will begin to understand that the "bent shape" of the water molecules causes them to link up into hexagons. Students could write an essay on the following question: Why do snowflakes tend to have a hexagonal shape? [Note: To reinforce students' abilities to imagine the shapes of molecules, ask them to visit the Molecular Shapes Flash Interactive. Encourage them to think about how the water molecule relies on its shape as it joins with others to create a crystal structure.]